Electro luminescence display device and method of testing the same

ABSTRACT

To increase the proportion of the perfects to the whole lot of final products and to reduce the cost for active matrix EL display devices by checking the operation of a TFT substrate before depositing an EL material. A capacitor for testing is connected to a drain terminal of a driving TFT in a pixel portion to observe charging and discharging of the capacitor. Whether the driving TFT is normal or not is judged by the observation, so that the rejects can be removed before the manufacturing process is completed.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to an electronic display (electrooptical device) formed by fabricating an EL (electro luminescence) on asubstrate. In particular, the present invention relates to a displaydevice using a semiconductor element (an element which uses asemiconductor thin film). Further, the present invention relates to anelectronic device using an EL display in a display portion and themethod of detecting the EL display.

[0003] The EL element herein refers to both an element that utilizeslight emission from a singlet exciton (fluorescence) and an element thatutilizes light emission from a triplet exciton (phosphorescence).

[0004] 2. Description of the Related Art

[0005] Recently, a technique for forming a thin film transistor(hereinafter, referred to as TFT) on a substrate has been remarkablydeveloped, and a development of its application to an active matrixdisplay device has been continuously made. In particular, TFTs using apolysilicon film can operate at high speed, because such TFTs have ahigher field effect mobility than TFTs using a conventional amorphoussilicon film. Therefore, the control of pixels, which has beenconventionally conducted by a driver circuit provided outside asubstrate, can be performed by a driver circuit provided on the samesubstrate on which the pixels are provided.

[0006] Such an active matrix display device includes various circuitsand elements formed on the same substrate. With this structure, theactive matrix display device provides various advantages such as reducedmanufacturing cost, reduced size of a display device, an increasedyield, and a reduced throughput.

[0007] Furthermore, an active matrix EL display device including an ELelement as a self-luminescent element has been actively studied. The ELdisplay device is also called Organic EL Display (OELD) or Organic LightEmitting Diode (OLED).

[0008] In contrast with the liquid crystal display device, the ELdisplay device is selfluminescent. The EL element has such a structurethat an EL layer is sandwiched between a pair of electrodes (anode andcathode). However, the EL layer has normally a lamination structure. Asa typical example of the lamination structures, a lamination structure“hole transport layer/light emitting layer/electron transport layer”proposed by Tang et al. of Eastman Kodak Company is cited. Thisstructure has an extremely high light emitting efficiency. For thisadvantage, most light emitting devices, which are currently under studyand development, employ this structure.

[0009] Furthermore, the light emitting device may have such a laminationstructure that a hole injection layer, a hole transport layer, a lightemitting layer and an electron transport layer are deposited on an anodeor a hole injection layer, a hole transport layer, a light emittinglayer, an electron transport layer, and an electron injection layer aredeposited on an anode in this order. Moreover, the light emitting layermay be doped with a fluorescent pigment or the like.

[0010] All layers formed between a cathode and an anode are referred togenerically as EL layers within this specification. The above statedhole injecting layer, hole transporting layer, light emitting layer,electron transporting layer, electron injecting layer, and the like aretherefore all contained within the EL layer.

[0011] A predetermined voltage is then applied to the EL layer havingthe above structure by a pair of electrodes, thus recombination of acarrier thus occurs in the light emitting layer, and light is emitted.Note that the emission of light by the EL element is referred to asdriving the EL element throughout this specification. Further, an ELelement formed by an anode, an EL layer, and a cathode is referred to asan EL element throughout this specification.

[0012] As a method of driving an EL display device, an analog drivingmethod (analog drive) can be given. The analog drive of an EL displaydevice is described with reference to FIGS. 10 and 11.

[0013]FIG. 10 shows a structure of a pixel portion of an EL displaydevice that is driven in an analog manner. Gate signal lines (G1 throughGy) to which a gate select signal from a gate signal line driver circuitis input are connected to a gate electrode of a switching TFT 1801included in each pixel. One of a source region and a drain region of theswitching TFT 1801 included in each pixel is connected to source signallines (also referred to as data signal lines) S1 to Sx to which ananalog video signal is input, whereas the other is connected to a gateelectrode of an EL driver TFT 1804 included in each pixel and acapacitor 1808 included in each pixel.

[0014] A source region and a drain region of the driver TFT 1804included in each pixel are connected to power source supply lines V1through Vx and to an EL element 1806, respectively. An electricpotential of the power source supply lines V1 through Vx is referred toas an power source electric potential. The power source supply lines V1through Vx are connected to the capacitors 1808 included in therespective pixels.

[0015] The EL element 1806 includes an anode, a cathode and an EL layersandwiched between the anode and the cathode. If the anode of the ELelement 1806 is connected to the source or the drain region of thedriver TFT 1804, the anode and the cathode of the EL element 1806 becomea pixel electrode and an opposing electrode, respectively. On the otherhand, if the cathode of the EL element 1806 is connected to the sourceor the drain region of the driver TFT 1804, the anode and the cathode ofthe EL element 1806 become an opposing electrode and a pixel electrode,respectively.

[0016] Note that the electric potential of the opposing electrode isreferred to as an opposing electric potential in this specification.Note also that an power source for imparting the opposing electricpotential to the opposing electrode is referred to as an opposingelectric power supply. The electric potential difference between theelectric potential of the pixel electrode and the electric potential ofthe opposing electrode is an EL driver voltage, and the EL drivervoltage is applied to the EL layer.

[0017]FIG. 11 shows a timing chart in the case where the EL displaydevice shown in FIG. 10 is driven in an analog manner. The period fromthe selection of one gate signal line until the selection of a next gatesignal line is called one line period (L). The period from the displayof one image to another image corresponds to one frame period (F). Inthe case of the EL display device shown in FIG. 10, since there are ygate signal lines, y line periods (L1 to Ly) are provided within oneframe period.

[0018] With the enhancement in resolution, the number of line periodswithin one frame period increases. As a result, the driver circuit mustbe driven at a high frequency.

[0019] An power source electric potential at the power source supplylines (V1 through Vx) is held constant, and an opposing electricpotential at the opposing electrodes is also held constant. The opposingelectric potential has a potential difference with the power sourceelectric potential to such a degree that a EL element 1806 emits light.

[0020] The gate signal line G1 is selected in the first line period L1by a gate signal input to the gate signal line G1 from the gate signalline driver circuit. Then an analog video signal is then input in orderto the source signal lines S1 to Sx. All of the switching TFTs 1801connected to the gate signal line G1 are in an ON state, and thereforethe analog video signal input to the source signal lines S1 to Sx isinput to gate electrodes of the driver TFTs 1804 through the switchingTFTs 1801.

[0021] The description here takes as an example a timing chart of thecase where the switching TFT 1801 and the driving TFT 1804 are bothn-channel TFTs. The switching TFT and the driving TFT may instead bep-channel TFTs, or one of them may be an n-channel TFT while the otheris a p-channel TFT.

[0022] In this specification, the TFT being turned ON means that thegate voltage of the TFT is changed such that the source-drain thereof isbrought into conductive state.

[0023] The amount of a current flowing through a channel formationregion of the driver TFT 1804 is controlled by a level of an electricpotential (voltage) of a signal input to the gate electrode of thedriver TFT 1804. Accordingly, the electric potential applied to thepixel electrode of the EL element 1806 is determined by the level of theelectric potential of the analog video signals input to the gateelectrode of the driver TFT 1804. Then, the EL element 1806 iscontrolled by the electric potential of the analog video signals to emitlight.

[0024] When the above-described operation is repeated to complete theinput of analog video signals to the source signal lines (S1 throughSx), the first line period (L1) terminates. One line period mayalternatively be constituted by the period until the completion of inputof the analog video signals to the source signal lines (S1 through Sx)and a horizontal blanking period. Then, a second line period (L2) startswhere a gate signal line G2 is selected by a gate signal. And, as in thefirst line period (L1), analog video signals are sequentially input tothe source signal lines (S1 through Sx) during the second line period.

[0025] When all gate signal lines (G1 through Gy) are selected in thismanner, all lines periods (L1 through Ly) are completed. The completionof all the line periods (L1 through Ly) corresponds to the completion ofone frame period. All pixels perform display during one frame period toform an image. One frame period may be alternatively constituted by allline periods (L1 through Ly) and a vertical blanking period.

[0026] The amount of light emitted by the EL element 1806 is thuscontrolled in accordance with the analog video signal, and gray scaledisplay is performed by controlling the amount of light emitted. Thismethod is namely a driving method referred to as an analog drivingmethod, gray scale display is performed by changing the electricpotential of the analog video signal input to the source signal lines.

[0027] In the conventional EL display device, the drain region of thedriving TFT 1804 in the pixel portion is connected only to the ELelement 1806 as shown in FIG. 10.

[0028] TFTs are formed on a substrate having an insulating surface inorder to constitute pixel TFTs (each formed of a switching TFT and adriving TFT) and driver circuits (including a source signal line drivingcircuit and a gate signal line driving circuit). An EL material is thendeposited and the driving TFT is electrically connected to an ELelement. The manufacturing steps prior to the step of depositing the ELmaterial are called herein TFT steps.

[0029] Before the EL material is deposited, the drain region of thedriving TFT in the conventional display device is thus in an open statefrom the design of the circuit. Whether a certain pixel TFT operatesnormally or not cannot be judged until the EL material is deposited tocomplete the display device and lighting test is performed on thecompleted device. Therefore, it is not until after the manufacturingprocess reaches the final step that a display device incapable of normaldisplay because of a defective pixel TFT can be found out. This isutterly a waste.

[0030] As described above, the conventional EL display device does notallow its pixel TFTs to be checked for their operation during the stepsprior to deposition of the EL material, thereby incurring a waste inmanufacturing cost.

SUMMARY OF THE INVENTION

[0031] The present invention has been made in view of the above problem,and an object of the present invention is therefore to provide an activematrix EL display device that allows its pixel TFTs to be checked fortheir operation before depositing an EL material.

[0032] In order to attain the object above, the inventors of the presentinvention have thought of cutting waste along the manufacture line bytesting driving TFTs and switching TFTs before depositing an EL materialand removing substrates having TFTs that are found to be defectivethrough the test (hereinafter referred to as the rejects) from thesubsequent step of depositing the EL material.

[0033] The structure of the EL display device of the present inventionis as described below.

[0034] In a first aspect of the present invention, an EL display deviceis provided which comprises a plurality of source signal lines, aplurality of gate signal lines, a plurality of power supply lines, aplurality of switching thin film transistors and a plurality of drivingthin film transistors which are all formed on an insulating substrate,comprising a testing capacitor which is set such that one end thereof isconnected to a drain region of each of the driving thin film transistorsand the other end thereof is connected to one of the gate signal lines,and characterized in that the power supply lines are led out of theinsulating substrate through switches.

[0035] In a second aspect of the present invention, an EL display deviceis provided characterized in that, the switches are provided for theplural power supply lines on one on one basis, and a driver circuit fordriving the switches successively is placed on the insulating substrate.

[0036] In a third aspect of the present invention, an EL display deviceis provided characterized in that the driver circuit for driving theswitches successively has some components shared with a source signalline driving circuit.

[0037] In a fourth aspect of the present invention, a method of testingan EL display device is provided, which comprises a plurality of sourcesignal lines, a plurality of gate signal lines, a plurality of powersupply lines, a plurality of switching thin film transistors and aplurality of driving thin film transistors which are all formed on aninsulating substrate, the driving thin film transistors each having adrain region that is connected to a testing capacitor, the methodcomprising the steps of: operating the driving thin film transistors tocharge the testing capacitor until it reaches and keeps a certain levelof electric potential; turning the driving thin film transistors OFF andthen setting the electric potential of the power supply lines to a leveldifferent from the electric potential of the testing capacitor; andleading the electric charges charged in the testing capacitor out foreach pixel through its associated power supply line, so that a change inelectric potential can be detected.

[0038] In a fifth aspect of the present invention, a computer, a videocamera, a head mount display, an image reproducing device, a portableinformation terminal characterized by using an EL display device.

BRIEF DESCRIPTION OF THE DRAWINGS

[0039] In the accompanying drawings:

[0040]FIG. 1 is a diagram showing the circuit structure of a pixelportion of a display device according to the present invention;

[0041]FIG. 2 is a block diagram of a display device according to thepresent invention;

[0042]FIG. 3 is a timing chart illustrating a method of driving adisplay device according to the present invention;

[0043]FIG. 4 is a diagram showing an embodiment of a testing drivercircuit in a display device according to the present invention;

[0044]FIG. 5 is a diagram showing an embodiment of an external testingcircuit for a display device according to the present invention;

[0045]FIGS. 6A and 6B are a top view of a display device according tothe present invention and a sectional view thereof, respectively;

[0046]FIG. 7 is a circuit diagram showing a source signal line drivingcircuit of a display device according to the present invention;

[0047]FIG. 8 is a top view of a latch of a display device according tothe present invention;

[0048]FIGS. 9A to 9E show electronic equipments employing a displaydevice of the present invention;

[0049]FIG. 10 is a circuit diagram of a pixel portion of a conventionaldisplay device;

[0050]FIG. 11 is a timing chart illustrating an analog driving methodfor a display device;

[0051]FIG. 12 is a diagram showing a cellular phone that employs thepresent invention; and

[0052]FIG. 13 is a diagram illustrating how to use the cellular phonethat employs the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0053] The structure of an EL display device of the present inventionand a method of testing the same will be described below.

[0054]FIG. 1 shows Embodiment Mode 1 of the present invention. Shown inFIG. 1 is a structural diagram of a pixel portion in the display deviceof the present invention.

[0055] Each pixel in the pixel portion comprises a power supply line(one of V1 to Vx), a source signal line (one of S1 to Sx), a gate signalline (one of G1 to Gy), a switching TFT 9102, a driving TFT 9106, acapacitor storage 9107, an EL element 9105 and a testing capacitor 9108.

[0056] The switching TFT 9102 shown in FIG. 1 has a double gatestructure. However, the switching TFT in each pixel of the displaydevice according to the present invention is not limited to a doublegate structure. It may take a single gate structure or a multi-gatestructure having three or more gates.

[0057] The driving TFT 9106 shown in FIG. 1 has a single gate structure.However, the driving TFT in each pixel of the display device accordingto the present invention may take a double gate structure or amulti-gate structure.

[0058] A drain region of the driving TFT 9106 is connected not only tothe EL element 9105 but also to the testing capacitor 9108. The testingcapacitor 9108 is interposed between the drain region of the driving TFT9106 and the gate signal line in this example. However, the structure ofthe present invention is not limited thereto and the testing capacitormay be placed between the drain region and another individual wiring.

[0059]FIG. 2 is a block diagram showing the display device of thepresent invention.

[0060] The display device comprises a source signal line driving circuit9201, gate signal line driving circuits 9202 and 9203, a testing drivercircuit 9204, switches 9205 and 9206, source signal lines 9207 to 9209,gate signal lines 9210 to 9212, pixel portion power supply lines 9213and 9214, a power supply line lead out terminal 9215 and an externaltesting circuit 9216.

[0061]FIG. 2 shows some of the source signal lines, the gate signallines, the power supply lines and the switches as representatives of therest. The number of these lines actually provided corresponding to thenumber of pixels that constitute the display device.

[0062] Unlike the prior art, the testing driver circuit 9204 is added tothe display device of the present invention. Also added are the switches9205 and 9206 that are controlled by the testing driver circuit 9204 andinterposed between the power supply line lead out terminal 9215 and thepixel portion power supply line 9213 and between the power supply linelead out terminal 9215 and the pixel portion power supply line 9214,respectively. The power supply line lead out terminal is connected tothe external testing circuit 9216.

[0063] The testing driver circuit 9204 is arranged independently in FIG.2. However, the source signal line driving circuit may also serve as thetesting driver circuit if the signal line driving circuit is of analogtype. (Not shown)

[0064] Next, a testing method used in the present invention will bedescribed.

[0065] The description will be given with reference to FIGS. 1 and 2.

[0066] Assumed here is a test of a substrate in a stage after the TFTsteps but before deposition of an EL material. The test may instead beconducted at other stages as long as wire connection has already beenestablished between the TFTs, and between the capacitors (including thecapacitor storage, the testing capacitor, etc.) and resistors or othercomponents which constitute the display device.

[0067] Note that, although the EL element 9105 is found in FIG. 1, theEL element is not deposited and hence the EL element 9105 is not formedyet at the time of the test described below.

[0068] The first step is to apply a voltage corresponding to a signal“Hi”, e.g. 10 V, to the power supply lines. Then successively runningthe driving circuits (the source signal line driving circuit 9201 andthe gate signal line driving circuits 9202 and 9203), the driving TFT9106 in each pixel is turned ON to write the 10 V voltage of the powersupply lines V1 to Vx in each testing capacitor 9108. Capacitance of thetesting capacitor is set from 0.05 pF to 1 pF.

[0069] The second step is as follows. The source signal line drivingcircuit 9201 and the gate signal line driving circuits 9202 and 9203 areoperated to turn every driving TFT 9106 in the pixel portion OFF. Thenthe electric potential of the power supply lines V1 to Vx is set to avoltage corresponding to a signal “Lo”, e.g., 0 V. The switches 9205 and9206 at this point are remained ON.

[0070] The power supply lines V1 to Vx in the pixel portion thus come tohave 0V.

[0071] The third step includes the following operations.

[0072] The driving TFT 9106 in each pixel is put in motion in accordancewith a timing illustrated in FIG. 3.

[0073] The timing chart of FIG. 3 shows the case where the switching TFTis an n-channel TFT and the driving TFT is a p-channel TFT. However, theswitching TFT may either be an n-channel TFT or a p-channel TFT and thesame applies to the driving TFT.

[0074] The source signal lines S1 to Sx are sequentially operated. Inthe illustration of FIG. 3, two source signal lines S1 and S2 representthe rest of the source signal lines and merely the operation for S1 andS2 will be described here. However, this operation is performed on allof the source signal lines S1 to Sx.

[0075] In a pixel whose source signal line has received input of asignal “Lo”, the driving TFT 9107 is turned ON when a signal “Hi” isinputted to the gate signal lines G1 to Gy successively.

[0076] If all of the power supply lines V1 to Vx are connected, wiringcapacitor is too large to detect the voltage. Therefore, in order tocheck the operation of the pixel TFTs, the switches to be connected tothe power supply lines have to be arranged such that one switch isallocated to one column of pixels.

[0077] One column of pixels here is of pixels having switching TFTsconnected to the same source signal line.

[0078] During the source signal line S1 is selected, a switch connectedto the power supply line V1 for supplying power to pixels whoseswitching TFTs have source regions connected to the source signal lineS1 is turned ON. Meanwhile, the switches connected to the power supplylines V2 to Vx that are associated with the other pixels are all turnedOFF.

[0079] The source signal line S2 is selected next, during which a switchconnected to the power supply line V2 for supplying power to pixelswhose switching TFTs have source regions connected to the source signalline S2 is turned ON. Meanwhile, the switches connected to the powersupply lines V1 and V3 to Vx that are associated with the other pixelsare all turned OFF.

[0080] In FIG. 3, denoted by T1 and T2 are signals for turning ON or OFFthe switch connected to the power supply line V1 and the switchconnected to the power supply line V2, respectively.

[0081] In this embodiment, if T1 and T2 are signals “Hi” and inputted topower supply lines, the switches connected to those power supply linesare turned ON. On the other hand, the switches are turned OFF if T1 andT2 are signals “Lo”.

[0082] When the driving TFT 9106 is turned ON in each pixel, electriccharges held in the testing capacitor 9108 are discharged to the powersupply lines V1 to Vx. The discharge gives a voltage to the power supplylines V1 to Vx.

[0083] This voltage is given as follows. The voltage generated isexpressed as V_(out) and is obtained from the equation 1:

V _(out)=10×C3/(C1+C2+C3)

[0084] where C1 is a wiring capacitance value of the power supply linesin the pixel portion, C2 is a capacitance up through the power supplyline lead out terminal 9215, and C3 is a capacitance of the testingcapacitor.

[0085] If C1=C2=10 pF and C3=0.1 pF, the voltage V_(out) is 0.05 V.

[0086] The voltage V_(out) is small and hence the detection thereofrequires the external testing circuit 9216 connected to the power supplyline lead out terminal 9215.

[0087] The voltage V_(out) is not generated if the pixel TFT tested isdefective because charge or discharge cannot be made successfully.

[0088] In the timing chart of FIG. 3, the voltage V_(out) is generatedin the power supply line lead out terminal 9215 every time a pixel isselected if the pixels are normal. If the pixel TFT is defective, it canbe found out as a lack in voltage signal as indicated by 9301.

[0089] In this way, the pixel TFTs can be tested by selecting all pixelsone by one.

[0090] Embodiments of the present invention will be descried below.

[0091] [Embodiment 1]

[0092] Embodiment 1 shows an example of the structure of a testingdriver circuit in a display device of the present invention.

[0093] In FIG. 4, the testing driver circuit comprises a shift register9402, NAND circuits 9403, 9404 and 9405, and buffer circuits 9406, 9407and 9408. The shift register is comprised of a DFF 9401. The buffercircuits are comprised of inverters.

[0094] Although FIG. 4 shows merely a portion of the testing drivercircuit which corresponds to three power supply lines, the actualtesting driver circuit has all the circuits that correspond to all ofthe power supply lines.

[0095] The buffer circuits 9406, 9407 and 9408 have outputs 9409, 9410and 9411, respectively. The outputs are connected to switches 9205,9206, . . . shown in FIG. 2 and further connected to pixel portion powersupply lines and a power supply line lead out terminal.

[0096] When a voltage “Hi” is inputted to an input terminal 9400 of theshift register 9402, the terminals 9409 to 9411 all outputs signalscorresponding to “Hi”, whereby all the switches are turned ON.

[0097] [Embodiment 2]

[0098] Embodiment 2 shows an example of the structure of an externaltesting circuit for a display device of the present invention.

[0099] In FIG. 5, an external testing circuit 9501 comprises a switch9502 for switching connections, an amplifier 9505 for detecting asignal, a voltage source 9503, a resistor 9504, etc.

[0100] The switch 9502 selects one connection out of connections withthree input terminals consisting of the voltage source 9503, a voltagesource 9508 and the amplifier 9505. The voltage source 9503 is for avoltage corresponding to a signal “Hi”, i.e., 10V. The voltage source9508 is for a voltage corresponding to a signal “Lo”, i.e., 0V. Theamplifier 9505 amplifies a signal.

[0101] Note that the voltages of the voltage sources 9503 and 9508 arenot limited to the values above but may be set to optimal values ifnecessary.

[0102] The power supply line lead out terminal of the substrate of thedisplay device to be tested is connected to an input 9507, and the testis conducted following the steps described in Embodiment Mode 1. In thetest, judgement is made by monitoring an output 9506 of the amplifier9505.

[0103] The amplifier 9505 used here has a voltage gain of about 10 to1000 folds, and amplifies a detection signal generated in the powersupply line to sense the signal. A desirable gain of the amplifier is onthe order of 100 folds.

[0104] This embodiment can be carried out in combination with Embodiment1 without restriction.

[0105] [Embodiment 3]

[0106] A driving TFT 108 in the present invention may be an n-channelTFT or a p-channel TFT. However, if an anode of an EL element 110 servesas a pixel electrode and a cathode thereof serves as an oppositeelectrode, a p-channel TFT is preferably used for the driving TFT 108.On the other hand, when the anode of the EL element 110 serves as theopposite electrode and the cathode thereof serves as the pixelelectrode, the driving TFT 108 is preferably an n-channel TFT.

[0107] This embodiment can be carried out in combination with Embodiment1 or 2 without restriction.

[0108] [Embodiment 4]

[0109] Embodiment 4 shows an example of manufacturing an EL displaydevice in accordance with the present invention. FIG. 6A is a topsurface diagram of an EL display device using the present invention. InFIG. 6A, reference numeral 4010 denotes a substrate, 4011 denotes apixel portion, 4012 denotes a source signal line driver circuit, and4013 a and 4013 b denote gate signal line driver circuits. Therespective driver circuits are connected to an external equipment viawirings 4014 a, 4014 b, 4015 and 4016 leading to an FPC 4017.

[0110] Note that in this embodiment, an example where the source signalline driver circuit 4012 functions as a testing driver circuit isdescribed, however, the present invention is not limited to thisstructure. The testing driver circuit may be provided apart from thesource signal line driver circuit.

[0111] A cover material 6000, a sealing material (also referred to as ahousing material) 7000, an airtight material (a second sealing material)7001 are provided at this time so as to surround at least the pixelportion, and preferably the driver circuit and the pixel portion.

[0112] Further, FIG. 6B is a cross sectional structure of an EL displaydevice of this embodiment, and a driver circuit TFT (note that a CMOScircuit in which an n-channel TFT and a p-channel TFT are combined isshown in the figures here) 4022 and a pixel portion TFT 4023 (note thatonly a driver TFT for controlling the electric current to the EL elementis shown in the figures here) are formed on a base film 4021 on thesubstrate 4010. Known structures (top gate structures or bottom gatestructures) may be used for these TFTs.

[0113] Note that a testing capacitor connected to a drain electrode ofthe driver TFT is not shown in FIG. 6B.

[0114] After completing the driver circuit TFT 4022 and the pixelportion TFT 4023 a pixel electrode 4027 made from a transparentconductive film for electrically connecting to a drain of the pixelportion TFT 4023 is formed on an interlayer insulating film (levelingfilm) 4026 made from a resin material. A compound of indium oxide andtin oxide (referred to as ITO) and a compound of indium oxide and zincoxide can be used as the transparent conductive film. An insulating film4028 is formed once the pixel electrode 4027 is formed, and an openportion is formed on the pixel electrode 4027.

[0115] Then, an EL layer 4029 is formed. A lamination structure of aknown EL material (hole injecting layer, hole transporting layer, lightemitting layer, electron transporting layer, and electron injectinglayer), or a single layer structure, may be used for the EL layer 4029.The structure is formed using a known technique. Further, there are lowmolecular weight materials and high molecular weight materials (polymermaterials) for the EL material. An evaporation method is used when a lowmolecular weight material is used, but it is possible to use a simplemethod such as spin coating, printing or ink-jet printing when a highmolecular weight material is used.

[0116] The EL layer 4029 is formed by evaporation using a shadow mask inthis embodiment. Color display becomes possible by forming lightemitting layers (a red color light emitting layer, a green color lightemitting layer, and a blue color light emitting layer) capable ofemitting light at different wavelength for each pixel using the shadowmask. In addition, a method of combining a color changing layer (CCM)and a color filter, and a method of combining a white color lightemitting layer and a color filter are available, and both may be used.Of course, a single color light emitting EL display device can also bemade.

[0117] After forming the EL layer 4029, a cathode 4030 is formed on theEL layer. It is preferable to remove as much moisture and oxygen aspossible from the interface between the cathode 4030 and the EL layer4029. Therefore, a method in which the EL layer 4029 and the cathode4030 are formed in succession within a vacuum, or in which the EL layer4029 is formed in an inert atmosphere and the cathode 4030 is thenformed without exposure to the atmosphere is necessary. In thisembodiment, the above film formation can be performed by using amulti-chamber method (cluster tool method) film formation apparatus.

[0118] Note that a lamination structure of a LiF (lithium fluoride) filmand an Al (aluminum) film is used as the cathode 4030 in thisembodiment. Specifically, a 1 nm thick LiF (lithium fluoride) film isformed by evaporation on the EL layer 4029, and a 300 nm thick aluminumfilm is formed on the LiF film. An MgAg electrode, which is a knowncathode material, may of course also be used. Then, the cathode 4030 isconnected to the wiring 4016 in a region denoted by reference numeral4031. The wiring 4016 is an electric power source supply line forapplying a predetermined voltage to the cathode 4030, and is connectedto the FPC 4017 through a conductive paste material 4032.

[0119] The cathode 4030 and the wiring 4016 are electrically connectedin the region shown by reference numeral 4031, and therefore it isnecessary to form contact holes in the interlayer insulating film 4026and the insulating film 4028. These contact holes may be formed duringetching of the interlayer insulating film 4026 (when the pixel electrodecontact hole is formed) and during etching of the insulating film 4028(when forming the open portion before forming the EL layer). Further,etching may also be performed together through to the interlayerinsulating film 4026 when etching the insulating film 4028. A contacthole having a good shape can be formed in this case provided that theinterlayer insulating film 4026 and the insulating film 4028 are formedby the same resin material.

[0120] A passivation film 6003, a filling material 6004, and the covermaterial 6000 are formed covering the surface of the EL element thusformed.

[0121] In addition, the sealing material 7000 is formed on the inside ofthe cover material 6000 and the substrate 4010 so as to surround the ELelement portion. The airtight material (the second sealing material)7001 is formed on the outside of the sealing material 7000.

[0122] The filling material 6004 functions as an adhesive for bondingthe cover material 6000. PVC (polyvinyl chloride), epoxy resin, siliconeresin, PVB (polyvinyl butyral) and EVA (ethylene vinyl acetate) can beused as the filling material 6004. A moisture absorption effect can bemaintained if a drying agent is formed on the inside of the fillingmaterial 6004, and therefore it is preferable to do so.

[0123] Furthermore, spacers may be included within the filling material6004. The spacers may be made from a powdered substance comprising amaterial such as BaO, giving the spacers themselves moisture absorbency.

[0124] In the case of providing the spacers, the passivation film 6003can relieve the spacer pressure. Further, a film such as a resin film,separate from the passivation film 6003, may also be formed forrelieving the spacer pressure.

[0125] Further, a glass plate, an aluminum plate, a stainless steelplate, an FRP (fiberglass-reinforced plastics) plate, a PVF (polyvinylfluoride) film, a mylar film, a polyester film, and an acrylic film canbe used as the cover material 6000. Note that when using PVB or EVA asthe filling material 6004, it is preferable to use a sheet having astructure in which several 10 of μm of aluminum foil is sandwiched by aPVF film or a mylar film.

[0126] Note that, depending upon the direction of light emitted from theEL elements (light emission direction), it may be necessary for thecover material 6000 to have light transmitting characteristics.

[0127] Further, the wiring 4016 is electrically connected to the FPC4017 through a gap between the sealing material 7000 and the airtightmaterial 7001, and the substrate 4010. Note that, although the wiring4016 is explained here, the other wirings 4014 a, 4014 b, and 4015 arealso electrically connected to the FPC 4017 through a gap between thesealing material 7000 and the airtight material 7001, and the substrate4010.

[0128] Note that the cover material 6000 is bonded after forming thefilling material 6004 in Embodiment 5, and that the sealing material7000 is attached so as to the side surface (exposed surface) of thefilling material 6004, but the filling material 6004 may also be formedafter attaching the cover material 6000 and the sealing material 7000. Afilling material injection port passing through the gap formed by thesubstrate 4010, the cover material 6000 and the sealing material 7000 isformed in this case. The gap is then placed in a vacuum state (equal toor less than 10⁻² Torr), and after immersing the injection port in atank containing the filling material, the pressure on the outside of thegap is made higher than the pressure within the gap, and the fillingmaterial fills the space.

[0129] This embodiment can be carried out in combination withEmbodiments 1 to 3 without restriction.

[0130] [Embodiment 5]

[0131] Embodiment 5 describes the structure of a source signal linedriving circuit in the case of employing a digital time gray scaledriving method instead of the analog gray scale driving method that hasbeen described in the example of the prior art.

[0132]FIG. 7 shows as a circuit diagram an example of the source signalline driving circuit used in this embodiment.

[0133] The present invention can adopt any one of the analog gray scaledriving method, the digital time gray scale driving method and thedigital area ratio gray scale driving method. The invention may alsotake a driving method that uses these gray scale methods in combination.

[0134] The source signal line driving circuit has a shift register 801,latches (A) 802 and latches (B) 803 which are arranged as shown in FIG.7.

[0135] In this embodiment, a pair of latches (A) 802 and a pair oflatches (B) 803 are associated with outputs to four source signal linesS_a to S_d. Therefore, four input lines VD are provided for digitalimage signals that are inputted from the external. The four input linesVD respectively receive signals to be inputted to the source signallines S_a to S_d.

[0136] A level shifter for changing the amplitude of the voltage of thesignal is not provided in this embodiment. However, the level shiftermay be provided at discretion.

[0137] A clock signal CLK, an inverted clock signal CLKB obtained byinverting the polarity of the CLK, a start pulse signal SP, and a drivedirection switching signal SL/R are inputted to the shift register 801respectively from the wirings shown in FIG. 7. A digital data signal VDis inputted to the latches (A) 802 from the wirings shown in FIG. 7. Alatch signal S_LAT and an inverted signal S_LATb obtained by invertingthe polarity of the S_LAT are inputted to the latches (B) 803respectively from the wirings shown in FIG. 7.

[0138] Details of the structure of the latches (A) 802 will be describedtaking as an example a portion 804 that is a part of the latches (A) 802associated with the source signal line S_a. The portion 804 that is apart of the latches (A) 802 has two clocked inverters and two inverters.

[0139]FIG. 8 shows a top view of the portion 804 that is a part of thelatches (A) 802. Denoted by 831 a and 831 b are active layers of TFTsthat constitute one of the inverters of the portion 804 that is a partof the latches (A) 802. Reference symbol 836 denotes a common gateelectrode of the TFTs constituting the one inverter. The other inverterof the portion 804 that is a part of the latches (A) 802 comprises TFTswhose active layers are denoted by 832 a and 832 b. On the active layers832 a and 832 b, gate electrodes 837 a and 837 b are provided. The gateelectrodes 837 a and 837 b are electrically connected to each other.

[0140] Denoted by 833 a and 833 b are active layers of TFTs thatconstitute one of the clocked inverters of the portion 804 that is apart of the latches (A) 802. On the active layer 833 a, gate electrodes838 a and 838 b are formed to provide a double gate structure. On theactive layer 833 b, the gate electrode 838 b and a gate electrode 839are formed to provide a double gate structure.

[0141] Denoted by 834 a and 834 b are active layers of TFTs thatconstitute the other clocked inverter of the portion 804 that is a partof the latches (A) 802. On the active layer 834 a, the gate electrode839 and a gate electrode 840 are formed to provide a double gatestructure. On the active layer 834 b, the gate electrode 840 and a gateelectrode 841 are formed to provide a double gate structure.

[0142] This embodiment can be carried out in combination with any one ofEmbodiments 1 through 4 without restriction.

[0143] [Embodiment 6]

[0144] According to an EL display device of the present invention, amaterial used for an EL layer of an EL element is not limited to anorganic EL material but may be an inorganic EL material. However, everyinorganic EL material at present is very high in drive voltage and hencea TFT to be used has to have a withstand voltage characteristic thatallows the TFT to withstand such a high drive voltage.

[0145] If an inorganic EL material of lower drive voltage is to bedeveloped in future. it may also be used in the present invention.

[0146] This embodiment can be carried out in combination with any one ofEmbodiments 1 through 5 without restriction.

[0147] [Embodiment 7]

[0148] An electronic display formed by implementing the presentinvention, in particular, an EL display device is utilized for variouselectronic equipments. Electronic equipments incorporating an electronicdisplay formed in accordance with this invention as a display mediumwill be explained as follows.

[0149] The following can be given as examples of such electronicequipments: a video camera; a television receiving machine; a digitalcamera; a head mounted display (a goggle type display); a game machine;a telephone; a car navigation system; a personal computer; an imagereproducing device; a portable information terminal (such as a mobilecomputer, a mobile telephone, or an electronic book). Examples of thoseelectronic equipments are shown in FIG. 9.

[0150]FIG. 9A illustrates a personal computer, which includes a mainbody 2001, a frame 2002, a display portion 2003, a key board 2004, orthe like. The EL display device of the present invention can be appliedto the display portion 2003 of the personal computer.

[0151]FIG. 9B illustrates a video camera, which includes a main body2101, a display portion 2102, an audio input portion 2103, operationswitches 2104, a battery 2105, an image receiving portion 2106, or thelike. The EL display device of the present invention can be applied tothe display portion 2102 of the video camera.

[0152]FIG. 9C illustrates a portion (the right-half piece) of a headmounted display, which includes a main body 2301, signal cables 2302, ahead mount band 2303, a display monitor 2304, an optical system 2305, adisplay portion 2306, or the like. The EL display device of the presentinvention can be applied to the display portion 2306 of the head mounteddisplay.

[0153]FIG. 9D shows an image reproducing device provided with arecording medium (specifically, a DVD reproducing device). The imagereproducing device comprises a main body 2401, a recording medium (CD,LD, DVD, or the like) 2402, operation switches 2403, a display portion(a) 2404, a display portion (b) 2405, etc. The display portion (a)mainly displays image information whereas the display portion (b) mainlydisplays character information. The EL display device of the presentinvention can be used for the display portions (a) and (b) of the imagereproducing device provided with a recording medium. Note that thepresent invention also can be applied to other image reproducingmachines provided with a recording medium, such as CD players and gamemachines.

[0154]FIG. 9E shows a portable (mobile) computer, which includes a mainbody 2501, a camera portion 2502, image receiving portion 2503,operation switches 2504, a display portion 2505, or the like. The ELdisplay device of the present invention can be applied to the displayportion 2505 of the portable (mobile) computer.

[0155] If the luminance of light emitted from EL materials is improvedin future, the present invention can be used in front or rear typeprojectors.

[0156] The electronic equipments in this embodiment can be carried outin combination with any one of Embodiments 1 through 6 withoutrestriction.

[0157] [Embodiment 8]

[0158]FIG. 12 shows an example in which an EL display device is used ina cellular phone.

[0159] The cellular phone comprises a casing A 1201, a casing B 1202 andan antenna 1205. A display portion 1200 and a microphone 1209 are formedon a surface A 1203 of the casing A 1201. Formed on a surface B 1204 ofthe casing B 1204 are a speaker 1206, operation keys 1207, a powerswitch 1208, etc.

[0160] An EL display device of the present invention can be used as thedisplay portion 1200 of the cellular phone.

[0161] The arrangement of the speaker 1206, the operation keys 1207, thedisplay portion 1200, the microphone 1209 and the power switch 1208 isnot limited to the one described above. The components can be formed onany part of the casing A 1201 and the casing B 1202. In FIG. 12, thecellular phone has two casing portions (the casing A 1201 and the casingB 1202) connected by a hinge (not shown) at one side. When the hinge isclosed, the surface A 1203 of the casing A 1201 can be laid on top ofthe surface B 1204 of the casing B 1202. To lay the surface A 1203 ofthe casing A 1201 on top of the surface B 1204 of the casing B 1202 isreferred to as folding the cellular phone in half.

[0162] In this example, the cellular phone can be used in a wayillustrated in FIG. 13. Since the speaker 1206 and the microphone 1209are set in different casings, it is possible to place the speaker 1206close to a ear 1211 and the microphone 1209 to a mouth 1212 with anadjustment of the angle between the surface A 1203 and the surface B1204 on which they are set. This structure has an advantage of blockingthe view of the mouth 1212 from the others while the user is speaking onthe phone. Moreover, the microphone 1209 being in proximity to the mouth1212 reduces the influence of noise and thereby improves the phonecommunication quality. Thus it is effective in reducing the number ofnoise filters in the cellular phone. With more operation keys 1207, thecellular phone can also serve as a portable information terminal.

[0163] As described above, the application range of the presentinvention is so wide that the invention can be applied to every field ofelectronic equipment. The electronic equipment of this embodiment can beobtained using any combination of structures of Embodiments 1 to 6.

[0164] With the structure above, a display device that allows its pixelTFTs to be tested before forming an EL layer can be provided. This makesit possible to remove the rejects before depositing an EL material,leading to a reduction in manufacturing cost.

What is claimed is:
 1. An electro luminescence display devicecomprising: a plurality of source signal lines; a plurality of gatesignal lines; a plurality of power supply lines; a plurality ofswitching thin film transistors; a plurality of driving thin filmtransistors; and a plurality of testing capacitors which are all formedon an insulating substrate, wherein one end of each of said testingcapacitors is connected to a drain region of each of the driving thinfilm transistors and the other end of said each testing capacitor isconnected to one of the gate signal lines, and wherein the power supplylines are led out of the insulating substrate through switches.
 2. Adevice according to claim 1 , wherein the switches are provided for theplural power supply lines on one on one basis, and wherein a drivercircuit for driving the switches successively is placed on theinsulating substrate.
 3. A device according to claim 2 , wherein thedriver circuit for driving the switches successively has some componentsshared with a source signal line driving circuit.
 4. A device accordingto claim 1 , further comprising an external testing circuit connectedthe power supply lines through said switches.
 5. A device according toclaim 1 , wherein capacitance of the testing capacitor is set from 0.05pF to 1 pF.
 6. A computer using an electro luminescence display deviceaccording to claim 1 .
 7. A video camera using an electro luminescencedisplay device according to claim 1 .
 8. A head mount display using anelectro luminescence display device according to claim 1 .
 9. An imagereproducing device using an electro luminescence display deviceaccording to claim 1 .
 10. A portable information terminal using anelectro luminescence display device according to claim 1 .
 11. Anelectro luminescence display device comprising: a pixel regioncomprising a plurality of source signal lines; a plurality of gatesignal lines; a plurality of power supply lines; a plurality ofswitching thin film transistors; a plurality of driving thin filmtransistors; a plurality of electro luminescence elements, a pluralityof storage capacitors; and a plurality of testing capacitors which areall formed over a substrate; and driver circuits comprising at least agate signal line driver circuit, a source signal line driver circuit,and a testing driver circuit which are all formed over said substrate,wherein one end of each of said testing capacitors is connected to adrain region of one of the driving thin film transistors and the otherend of said each testing capacitor is connected to one of the gatesignal lines, and wherein the power supply lines are connected to saidtesting driver circuit through switches.
 12. A device according to claim11 , further comprising an external testing circuit connected the powersupply lines through said switches.
 13. A device according to claim 11 ,wherein capacitance of the testing capacitor is set from 0.05 pF to 1pF.
 14. A computer using an electro luminescence display deviceaccording to claim 11 .
 15. A video camera using an electro luminescencedisplay device according to claim 11 .
 16. A head mount display using anelectro luminescence display device according to claim 11 .
 17. An imagereproducing device using an electro luminescence display deviceaccording to claim 11 .
 18. A portable information terminal using anelectro luminescence display device according to claim 11 .
 19. A methodof testing an electro luminescence display device comprising the stepsof: operating a thin film transistor to charge a testing capacitor untilit reaches and keeps a certain level of electric potential; turning thedriving thin film transistor OFF and then setting the electric potentialof the power supply lines to a level different from the electricpotential of the testing capacitor; leading the electric charges chargedin the testing capacitor out for each pixel through its associated powersupply line; and detecting a change in the electric potential.
 20. Amethod of testing an electro luminescence display device comprising thesteps of: applying a voltage corresponding to a signal “Hi” to the powersupply lines; turning a driving thin film transistor in each pixel ON towrite the voltage of the power supply lines in each testing capacitor;turning said driving thin film transistor OFF so that an electricpotential of the power supply lines is set to a voltage corresponding toa signal “Lo”; and then turning said driving thin film transistor ON,thereby electric charges held in the testing capacitor are discharged tothe power supply lines; and detecting the discharged voltage using atesting circuit connected to the power supply line lead out terminal.